The present invention relates to a new and novel process for the biological purification of waste water containing nitrogen compounds, in one or more steps, in the last of which the waste water is denitrified, with additional BOD being added to this denitrification step.
Waste water contains not only organic compounds, but also nitrogen compounds, such as, e.g., ammonia and nitrate. These compounds must be removed thoroughly from the waste water. The purification is effected biologically by means of bacteria. To accomplish this, use is made of the biological processes of denitrification and optionally nitrification, that are parts of the nitrogen cycle in nature. The denitrification process takes place in an anaerobic atmosphere, i.e., poor in oxygen, and it requires organic material. The organic material is expressed as biological oxygen demand, or as it is commonly referred, BOD. This is the amount of oxygen consumed by the bacteria when the organic material is oxidized by the bacteria.
In normal purifying installations this oxidation is effected by means of oxygen in the air, which is usually mechanically dissolved in the water. If no dissolved oxygen is present in the water during the denitrification reaction, the oxygen from the nitrate or nitrite is used as the oxidizing agent for the organic material. In the nitrification reaction ammonia is oxidized biologically into nitrite or, further, into nitrate. This reaction is effected by other bacteria than those effecting the denitrification reaction. Moreover, the atmosphere is rich in oxygen, i.e., aerobic. Hence, the entire purifying process must be effected in steps.
The simplest embodiment is a single denitrification in which waste water containing only nitrates as nitrogen compounds is purified in the above way.
If the impurities in the nitrogen-containing waste water mainly consist of nitrogen, both in the reduced and in the oxidized form, with comparatively little biologically degradable material, use is mostly made of a nitrification step followed by a denitrification step.
A third step may be used if the waste water contains both nitrogen compounds and biologically degradable organic material. This embodiment has been described in Netherlands Patent Application No. 72.14701. According to this embodiment, the first step is a denitrification step to make the BOD and nitrite plus nitrate in the waste water react. Next there is an oxidation step in which ammonia is oxidized into nitrite or nitrate. After these two steps, only nitrate and nitrite remain, which have to be removed in the third step, the second denitrification. The removal of these nitrates and nitrites will require BOD which must be added separately.
As very stringent demands are made on waste water discharged into surface water, virtually all nitrite and nitrate must be removed in the final step. This may be accomplished by adding excess BOD. One disadvantage of adding excess BOD is that, after the final step which is the (second) denitrification, the water will again contain BOD. This is not acceptable as the discharged water should not contain BOD. Therefore, a last or oxidizing step is necessary. By following this process, the requirements for discharged waster water can be met at any rate of treatment.
Obviously, the last oxidizing step depends on the amount of excess BOD present in the final denitrification step. Prior to this invention, it has not been possible to control the addition of BOD in this step to meet the requirements for discharging nitrite, nitrate and BOD in the discharged treated waste water without omitting the last, oxidizing step.